Toroid Chokes and Baluns

Commonly published information tells us winding a choke or 1:1 balun on a
toroid with this special winding technique increases common mode impedance
(choking isolation) of an 11-turn balun on a pair of #65 material ferrite cores:

The common mode rejection sweep, assuming a 50-ohm common mode source and
load is:

(pardon the camera jitter)

This sweep shows the following suppression using split winding method:

15.71 MHz

37.1MHz

134.8MHz

157 MHz

-33dB

-46dB (peak performance)

-19dB

-3dB (minimum rejection)

The same core was rewound using a normal parallel winding:

The sweep looked like this:

Common mode rejection, compared to the more complex winding, was:

Frequency

15.71 MHz

37.1MHz

134.8MHz

157 MHz

Split winding

-33dB

-46dB (peak performance)

-19dB

-3dB (minimum performance)

Normal winding

-47dB (peak performance)

-41dB

-4.6dB (minimum rejection)

-11dB

Conclusion

Shunt capacitance
is not always a bad
thing! While the
results vary with
the core and number
of turns, the more-difficult
"split winding" will
often just shift peak performance
points higher in frequency. Performance in the HF range and lower
could actually
be decreased with the extra effort resulting in a poorer HF balun.

Common mode impedance,
if the impedance is
primarily inductive,
increases when a slight amount of parallel capacitance is added.
Shunt capacitance does
not always, as is
often claimed, reduce performance by "leaking"
RF around a choke.

The rule is simple. With any parallel L and C (neglecting losses), impedance
always increases unless the parallel capacitive reactance (in ohms) is less than
half the value of the inductance (in ohms).
With a choke winding
impedance of j300
with an added
parallel -j150
capacitive
reactance, the
combined impedance
would be -j300.
Impedance is the same
value, only the sign
changes! Any value
of capacitive shunt reactance higher than
-j150 (less
capacitance value) would actually increase choke isolation!

There are many "improvements" we just blindly accept from reading
articles, because on the surface they seem to make sense. This dispels one of
the myths about baluns.

Another common myth is the widely accepted claim moving a choke balun to a
lower common-mode impedance point of a circuit, such as the input of a tuner,
improves balance or balun performance in the system. If you think baluns at the
input of tuners are a good idea, go to this
W7EL link and read the section on current baluns. You might be
surprised!